Polish cache sample

Update single cache sample with all supported cache providers. Add README
to shortly explain how to get started with them.

See gh-2633

spring-boot-samples/spring-boot-sample-cache/src/main/java/sample/CountryRepository.java

@@ -0,0 +1,34 @@
+/*
+ * Copyright 2012-2015 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package sample;
+
+import javax.cache.annotation.CacheDefaults;
+import javax.cache.annotation.CacheResult;
+
+import org.springframework.stereotype.Component;
+
+@Component
+@CacheDefaults(cacheName = "countries")
+public class CountryRepository {
+
+	@CacheResult
+	public Country findByCode(String code) {
+		System.out.println("---> Loading country with code '" + code + "'");
+		return new Country(code);
+	}
+
+}